You probably have a 250w. kit? It will only have a "walk assist" throttle function as Yose sell these as UK and EU "legal". It probably doesn't allow you to increase the top speed either? I have a rear hub kit and found the same as you. I bought an 18 amp controller that Yose sell as a "350w. controller" and fitted it instead of the 15 amp one that came with the kit. This "unlocked" the throttle to allow a choice of "full" or "walk assist". There was also a significant hill climbing improvement as well. It didn't allow speed setting, and I suspect that this is a restriction set by the C500 display. If you want to increase the top speed you will probably have to buy another C500 display which Yose sell separately. I didn't bother.
I also have a 350w. rear hub kit on another bike, which has full throttle and top setting speed up to 30-32Km/hr.
I suspect your wife's bike either has a 350w. kit, or perhaps Yose used to supply 250w. kits without the UK/EU restrictions.
Hope this helps!
